A Navigen founder, Dean Li, MD, PhD, has identified a novel endothelial receptor, Robo4. Robo4 is highly expressed in the retinal endothelium, and activation of this receptor by its endogenous ligand Slit-2 reduces VEGF-induced retinal vascular leak and oxygen induced retinopathy. In other organ systems, activation of Robo4 reduces tissue edema induced by inflammatory cytokines. Mechanistic studies have revealed that Robo4 blocks signaling from VEGF and other cytokines. Of importance Slit-Robo4 preferentially inactivates VEGF-induced permeability and sprouting without having any direct effects on the ability of VEGF to act as an endothelial proliferation or survival factor. Thus, Robo4 is an endogenous endothelial system that counterbalances specific downstream pathways of the VEGF receptor. These preliminary studies provide the proof-of-concept that activating the Robo4 receptor will stabilize retinal vessels and prevent pathologic edema and angiogenesis. Specific Aim 1: Generate recombinant Robo4 peptide agonists. Specific Aim 2: Examine biologic activity of Robo4 peptide agonists Specific Aim 3: Determine the efficacy of Robo4 agonists in murine models of retinal and choroidal vascular disease. At the completion of this Phase I grant application, Navigen will identify recombinant Robo4 peptide agonists that will be extensively studied in a subsequent Phase II SBIR. During this Phase II, we anticipate initiating detailed pharmacokinetic experiments along with direct comparisons between Robo4 agonist, anti VEGF strategies, and combination therapies. The Phase I and II grant applications will form the scientific foundation for advancing Robo4 agonists into clinical development and testing. . PUBLIC HEALTH RELEVANCE: Navigen Pharmaceutical will develop a novel class of protein therapies for treating retinal and choroidal vascular diseases. These diseases include retinopathy, diabetic macular edema and the wet form of age-related macular degeneration. We will use this Phase I SBIR to generate a candidate biologic that Navigen will advance toward clinical development during a subsequent Phase II for this SBIR. [unreadable] [unreadable] [unreadable]